A popular microwave transistor technology is MESFET technology. A common feature of this technology is that a Schottky barrier metal is used as the gate. The channel will typically be a doped GaAs semiconductor layer which overlies a semi-insulating substrate. Leakage along the surface from gate to drain is a common problem, and since reduction of series resistance in the channel is also highly desirable, the gate is commonly recessed. That is, the gate Schottky-barrier metallization is not deposited directly on the top surface of the semiconductor, but a recess is etched before the gate is deposited. This means that the electron population in the channel is slightly removed from the adverse effects of the surface states normally found at the semiconductor surface, and also means that the surface leakage path from gate to drain is longer. This technology suffers from: several very important limitations. The output power capability of a MESFET is limited by the gate-drain breakdown voltage and the conduction current through the channel. To improve the breakdown voltage, either a low carrier concentration buffer layer between the gate metal and the channel, or a graded channel approach can be used. By employing either an insulating or a semi-insulating buffer layer, the breakdown voltage can be greatly increased, due to the much higher breakdown field of the layer, while the current level is maintained. This should result in a device with improved output power. Unfortunately, it has proven difficult to fabricate metal-insulator-semiconductor or insulated gate FET from III-V compound semiconductors. This is largely due to the large lattice mismatch at the insulator interface and the difficulty in growing a good oxide layer.
There are FETs with GaAs (which is low doped or undoped) and highly doped AlGaAs, referred to (among other names) as HEMTs. The HEMT device structure has a GaAs channel layer which is either undoped or very lightly doped, under a doped AlGaAs layer. This provides very high channel mobilities, but results in very low current levels and high parasitic resistances. These devices were primarily intended for low voltage operation. HEMT devices are extremely sensitive to the quality of the interface between the GaAs and AlGaAs layers. In a HEMT structure, the active carrier population is very narrowly confined to a shallow layer underneath this heterojunction. This means that any degradation in the quality of this interface will drastically degrade the device characteristics.
A type of MISFET, using an undoped AlGaAs layer as an insulator over an n-type GaAs channel, has shown to be more applicable for microwave uses. The high breakdown field of the wide bandgap AlGaAs results in a very high gate breakdown voltage and a low prebreakdown gate leakage current. The presence of the gate insulator also reduces the gate capacitance. Moreover, the electron density in the channel is not all concentrated next to the heterojunction, which means that the series resistance of the channel is low, and also means that channel mobility will not be degraded by a less-than-perfect interface at the heterojunction.